There is a constant interest in protecting security or value documents against counterfeiting and unauthorized reproduction. This is preferably done by using a public recognition security feature, i.e. a security feature which can be easily determined without additional aids. A well known class of public security features are watermarks. These are normally easily recognizable in transmitted light, and display a predetermined motif or value.
Another example of a public security feature/device is security threads. Said threads are normally made of a plastic material and inserted into the paper during paper making. These threads are normally quite narrow and are visible as a dark line when the security document is viewed in transmitted light. Further, metallized security tapes or threads are known, which often are provided with a hologram that shows different visual impressions, such as colour effects or information, from different viewing angles. Normally, at least a portion of the security thread is arranged on the surface of the security document, such that it is easily visible.
Many of the above listed security features were previously an obstacle to counterfeiters, but are today more easily copied. Hence, there is a need for an alternative public security feature which is difficult to copy by known counterfeiting techniques.